Calluses are hard, rough areas of skin that are often yellowish in colour. They can develop on any part of the foot, but mostly on pressure areas.
Calluses develop when the skin rubs against something, such as a bone, a shoe or the ground. They often form over the ball of your foot because this area takes most of your weight when you walk.
Activities that put repeated pressure on the foot, such as running or walking barefoot, can cause calluses to form as well the reasons listed above.